A TFT-LCD (Thin Film Transistor Liquid Crystal Display) has been developed rapidly in recent years due to its advantages such as low power consumption, lightness and thinness, ease of use, high brightness, high contrast and high response speed, which occupies a leading position in a current flat panel display market. Meanwhile, in order to meet market demands, the TFT-LCD is developing towards high resolution, lightness and thinness, low power consumption and high quality. The display screen uniformity, high resolution, no interaction or the like are key requirements of a high quality TFT-LCD. A magnitude of a leakage current (Ioff) is one of important parameters affecting performance of an LCD screen. Too high leakage current will affect switching features of the TFT, resulting in display uneven, whitening, interaction and other display defects of the TFT-LCD. However, an active layer is quite sensitive to light, that is, even if the active layer is subject to irradiation of weak light, a quite large leakage current will generate in the active layer. With the increase of light intensity and extension of irradiating time, characteristics of the TFT device will sharply decline, resulting in deterioration of the display quality.